Request ID implementation - fixes #191

[mstoykov]

• [x] generation from the request counter and the started time of the app - the size 2^64 or 16 characters
• [x] optional configurable application id prefix to the request id
• [x] the id is logged with the access log and nowhere else by default - no configuration to remove it, or ability to print it with additional info before that.
• [x] all handlers have been updated
• [x] utility method to append to the current id in the context of the handler. Used by flv/mp4/cache handlers to append what byte range they are requesting.
• [ ] could the same be used with proxy on retry
• [ ] No HTTP header with the request id is added.

[ironsmile]

"No header with the request id is"?